Dental mill blanks

ABSTRACT

A dental mill blank comprising a resin and a filler, wherein the blank is fabricated such that it passes a Thermal Shock Test. The mill blank is substantially free of cracks and discontinuities. Further, the blank may have superior cuttability and hardness.

FIELD OF THE INVENTION

[0001] This invention is related to polymeric based mill blanks that aresubstantially free of cracks and are suitable for use in fabricatingdental and medical prostheses by CAD/CAM (computer-aideddesign/computer-aided machining) procedures.

BACKGROUND OF THE INVENTION

[0002] The art of fabricating custom-fit prosthetics in the medical anddental fields is well-known. Prosthetics are replacements for tooth orbone structure; examples include restoratives, replacements, inlays,onlays, veneers, full and partial crowns, bridges, implants, posts, etc.Currently, most prostheses in dentistry are either made by hand by adental practitioner while the patient is in the dental chair, or by anindependent laboratory who is capable of such fabrication.

[0003] Materials used to make the prostheses typically include gold,ceramics, amalgam, porcelain and composites. For dental restorative worksuch as fillings, amalgam is a popular choice for its long life and lowcost. Amalgam also provides a dental practitioner the capability offitting and fabricating a dental filling during a single session with apatient. The aesthetic value of amalgam, however, is quite low, as itscolor drastically contrasts to that of natural teeth. For large inlaysand fillings, gold is often used. However, similar to amalgam, goldfillings contrast to natural teeth hues. Thus, dental practitioners areincreasingly turning to ceramic or polymer-ceramic composite materialswhose color can be matched with that of the tooth.

[0004] The conventional procedure for producing dental prostheticstypically requires the patient to have at least two sessions with thedentist. First, an impression is taken of the dentition using anelastomeric material from which a cast model is made to replicate thedentition. The prosthetic is then produced from the model using metal,ceramic or a composite material. A series of steps for proper fit andcomfort then follows. Thus, fabrication of custom prostheses involvesintensive labor, a high degree of skill and craftsmanship, and lengthytimes (1-2 days). Alternatively, a practitioner may opt for a sinteredmetal system that may be faster. However, those procedures are stilllabor intensive and complicated.

[0005] In recent years, technological advances have provided computerautomated machinery capable of fabricating prostheses using minimalhuman labor and drastically lower work time. This is frequently referredto as “digital dentistry,” where computer automation is combined withoptics, digitizing equipment, CAD/CAM (computer-aided design/computeraided machining) and mechanical milling tools. Examples of such acomputer-aided milling machine include the CEREC 2™ machine supplied bySiemens (available from Sirona Dental Systems; Bensheim, Germany) VITACELAY™, (available from Vita Zahn Fabrik; Bad Sackingen, Germany)PRO-CAM™ (Intra-Tech Dental Products, Dallas, Tex.) and PROCERAALLCERAM™ (available from Nobel Biocare USA, Inc.; Westmont, Ill). U.S.Pat. Nos. 4,837,732, 4,575,805 and 4,776,704 also disclose thetechnology of computer-aided milling machines for making dentalprostheses. These machines produce dental prostheses by cutting,milling, and grinding the near-exact shape and morphology of a requiredrestorative with greater speed and lower labor requirements thanconventional hand-made procedures.

[0006] Fabrication of a prostheses using a CAD/CAM device requires a“mill blank,” a solid block of material from which the prosthetic is cutor carved. The mill blank is typically made of ceramic material. U.S.Pat. No. 4,615,678 discloses a blank adapted for use in machinefabrication of dental restorations comprising a ceramic silica material.There exist various mill blanks available commercially, including VITACELAY™ porcelain blanks Vita Mark II Vitablocks™ and VITA IN-CERAM™ceramic blanks (both available from Vita Zahn Fabrik; Bad Säckingen,Germany). Machinable micaceous ceramic blanks (e.g. Corning MACOR™blanks and Dentsply DICOR™) are also known in the art.

SUMMARY OF THE INVENTION

[0007] The invention provides mill blanks for making dental prostheticscomprising a polymeric resin and a filler, wherein the mill blank issubstantially free of cracks, or fissures, and able to withstand aThermal Shock Test, a test that exposes the existence of internalstresses in the mill blank, which can lead to cracking of the materialbefore or during the milling operation or during clinical use of theultimate prosthesis. Preferably, the mill blank of the present inventionis also substantially free of material discontinuities larger than about1 millimeter. The mill blank's surprising ability to pass a ThermalShock Test is a result of the relief of stress created during the curingprocess or proper low stress curing wherein little or no stress isactually created in the blank. Preferably low stress cure is performedby slow light curing methods. Heat treatment following a fast cure hasalso been surprisingly found to minimize internal stresses and providethe mill blank the same ability to pass the Thermal Shock Test.

[0008] By careful selection of the resin and filler, additionaldesirable material properties may be achieved, including superiorcuttability and hardness over commercially available blanks. Preferredresins arc free radically curable, cationically curable, or acombination thereof. Preferred fillers for the invention are those thathave been derived by sol-gel process.

DESCRIPTION OF THE INVENTION

[0009] Physical properties such as hardness and brittleness of ceramicslimit the usefulness as dental prosthetics. Metals also have theirshortcomings, as they are not aesthetic and may cause concern regardingallergic reactions and the like. Thus, it would be advantageous to havea prosthetic made from a strong and durable material, where the materialwould be suitable for use in simple and economical devices such asexisting CAD/CAM manufacturing equipment.

[0010] The present invention focuses on mill blanks made of highlyfilled composite material, suitable for use in fabricating dentalprostheses, preferably using precision manufacturing equipment, such asCAD/CAM milling devices.

[0011] The blanks of the present invention display excellent performancein many characteristics important for dental or medical use, includingcompressive strength, diametral tensile strength, flexural strength,fracture toughness, hardness, resistance to wear, wear on opposingdentition, durability, polishability, polish retention, esthetics,thermal expansion, visual opacity, x-ray opacity, impact strength,chemical durability, biocompatibility, modulus, shelf life, patientcomfort, ease-of-use, and structural integrity.

[0012] A “composite” material refers to a hardenable (or hardened)composition containing at least in part, a polymerizable (orpolymerized) resin(s), filler particles of one or more types, apolymerization initiator, and any desired adjuvants. Composites of thepresent invention can be multiple- or one-part compositions wherepolymerization may be initiated by a variety of means including heat,light, radiation, e-beam, microwave, or chemical reaction.

[0013] It has been surprisingly found that a mill blank made ofcomposite material provides certain advantages and appealing featuresover ceramic and porcelain blanks. Careful selection of the combinationof the components provides improved cuttability performance.“Cuttability”, as used herein, is a property of a mill blank of thepresent invention, characterized by how well a blank responds to contactfrom a cutting tool. For example, a measurement may be performed bymeasuring the depth of a cut made by a cutting tool when the tool isapplied with a constant force for a fixed period of time. Preferably,the cuttability value of a mill blank is established by a standard testdescribed herein, where the Cuttability Value is determined bycomparison to a standard material.

[0014] It has also been surprisingly found that careful selection of theresin, filler and adjuvants provides an advantageous capability of thecomposite to be loaded with substantially high amounts of filler. Thisfiller loading translates into improved durability, wear, and hardnessof the composite mill blank. The addition of filler to a compositionprovides desirable levels of viscosity for material processing andstrength for durability of the finished product. “Wear”, as used herein,is also a property of a mill blank of the present invention that can becharacterized by compressive strength and diametral tensile strength.Hardness can be characterized by a Barcol Hardness measurement. It isdesirable for a dental prosthetic to have a high resistance to wear anda high degree of hardness in order for it to maintain its intended shapeand integrity as well as be useful in the oral environment. However, itis also desireable that the prosthetic material not unduly wear opposingor surrounding dentition.

[0015] A further advantage the present invention has over ceramic millblanks is the ease of finishing. A practitioner would have the ability,if necessary, to repair or modify a prosthetic made from the presentinvention's composite composition much more easily than if the repairhad to be made on a ceramic or porcelain prosthetic. Ideally, likematerials would be used to repair a prosthetic in the oral environment,materials appropriate for repairing the instant prosthetic may be curedby radiant energy within the oral environment. In contrast, ceramicsrequire firing and sintering at extremely high temperatures (typicallygreater than 700° C.) and therefore a repair material made of ceramic isnot useful in the mouth.

[0016] The polymeric resin and filler of the present invention arepreferably selected such that the resulting mill blank has a BarcolHardness that is greater than or equal to the Barcol Hardness of a FumedSilica Mill Blank Standard. More preferably, the mill blank has a BarcolHardness that is about 5% greater than the Barcol Hardness of a FumedSilica Mill Blank Standard, and most preferably about 15% greater.Preferably, the polymeric resin and filler of the present invention areselected such that the Cuttability Value is about 30% greater than theCuttability Value of a Fumed Silica Mill Blank Standard, more preferably50% greater, and most preferably 100% greater. The Fumed Silica MillBlank Standard is a mill blank made from bis-GMA TEGDMA resin loadedwith silane treated fumed silica filler, such as the filler availableunder the trade name AEROSIL OX50 (Degussa Corporation, PigmentsDivision, Teterboro, N.J.). The fumed silica filler has an averageprimary particle size of 40 nanometers (nm), a surface area of 50±15m²/g as measured by DIN 66131, pH value of 3.7-4.7 via ASTM D1208,purity of greater than 99.8% SiO₂ and has a tap density of approximately130 g/l per ISO 787/×1 synthesized via continuous flame hydrolysis ofSiCl₄.

[0017] As used herein, “curable” and “polymerizable” are usedinterchangeably.

[0018] Polymerizable resins suitable for use in the dental compositemill blank of the present invention are hardenable organic resins havingsufficient strength, hydrolytic stability, and non-toxicity to renderthem suitable for use in the oral environment. Preferably, the resin ismade from a material comprising a free radically curable monomer,oligomer, or polymer, or a cationically curable monomer, oligomer, orpolymer, or both. Alternatively, the resin may be made from a materialcomprising a monomer, oligomer or polymer comprising both a freeradically curable functionality and a cationically curablefunctionality.

[0019] A particularly preferred polymerizable resin for use in thepresent invention is a mixture of two free radically curable monomers,namely, diglycidylmethacrylate of Bisphenol A (frequently referred to as“Bis-GMA”) and triethyleneglycol dimethacrylate (frequently referred toas “TEGDMA”). Such a material is available commercially under the tradename 3M Restorative™ Z100 (3M Co., St. Paul, Minn.). This particularresin creates unexpectedly preferred cutting and milling characteristicsduring the production of a dental prosthetic.

[0020] Other preferred polymerizable resins containing free radicallycurable functionalities include acrylates and methacrylates commonlyused in contemporary dental composites e.g.2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]propane (bisGMA);triethyleneglycol dimethacrylate (TEGDMA);2,2-bis[4-(2-methacryloyloxyethoxy)-phenyl]propane (bisEMA); 2-hydroxyethyl methacrylate (HEMA); urethane dimethacrylate (UDMA) andcombinations thereof.

[0021] Resins made from cationically curable material suitable for usein the present invention include epoxy resins. Epoxy resins impart hightoughness to composites, a desirable feature for composite mill blanks.Epoxy resins may optionally be blended with various combinations ofpolyols, methacrylates, acrylates, or vinyl ethers. Preferred epoxyresins include diglycidyl ether of bisphenol A (e.g. EPON 828, EPON 825;Shell Chemical Co.), 3,4-epoxycyclohexylmethyl-3-4-epoxy cyclohexenecarboxylate (e.g. UVR-6105, Union Carbide), bisphenol F epoxides (e.g.GY-281; Ciba-Geigy), and polytetrahydrofuran.

[0022] As used herein, “cationically active functional groups” is achemical moiety that is activated in the presence of an initiatorcapable of initiating cationic polymerization such that it is availablefor reaction with other compounds bearing cationically active functionalgroups. Materials having cationically active functional groups includecationically polymerizable epoxy resins. Such materials are organiccompounds having an oxirane ring, i.e., a group of the formula

[0023] which is polymerizable by ring opening. These materials includemonomeric epoxy compounds and epoxides of the polymeric type and can bealiphatic, cycloaliphatic, aromatic or heterocyclic. These materialsgenerally have, on the average, at least 1 polymerizable epoxy group permolecule, preferably at least about 1.5 and more preferably at leastabout 2 polymerizable epoxy groups per molecule. The polymeric epoxidesinclude linear polymers having terminal epoxy groups (e.g., a diglycidylether of a polyoxyalkylene glycol), polymers having skeletal oxiraneunits (e.g., polybutadiene polyepoxide), and polymers having pendentepoxy groups (e.g., a glycidyl methacrylate polymer or copolymer). Theepoxides may be pure compounds or may be mixtures of compoundscontaining one, two, or more epoxy groups per molecule. The “average”number of epoxy groups per molecule is determined by dividing the totalnumber of epoxy groups in the epoxy-containing material by the totalnumber of epoxy-containing molecules present.

[0024] These epoxy-containing materials may vary from low molecularweight monomeric materials to high molecular weight polymers and mayvary greatly in the nature of their backbone and substituent groups.Illustrative of permissible substituent groups include halogens, estergroups, ethers, sulfonate groups, siloxane groups, nitro groups,phosphate groups and the like. The molecular weight of theepoxy-containing materials may vary from about 58 to about 100,000 ormore.

[0025] Useful epoxy-containing materials include those which containcyclohexane oxide groups such as epoxycyclohexanecarboxylates, typifiedby 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate,3,4-epoxy-2-methylcyclohexylmethyl-3,4-epoxy-2-methylcyclohexanecarboxylate, and bis(3,4-epoxy-6-methylcyclohexylmethyl) adipate. For amore detailed list of useful epoxides of this nature, reference is madeto the U.S. Pat. No. 3,117,099, which is incorporated herein byreference.

[0026] Blends of various epoxy-containing materials are alsocontemplated. Examples of such blends include two or more weight averagemolecular weight distributions of epoxy-containing compounds, such aslow molecular weight (below 200), intermediate molecular weight (about200 to 10,000) and higher molecular weight (above about 10,000).Alternatively or additionally, the epoxy resin may contain a blend ofepoxy-containing materials having different chemical natures, such asaliphatic and aromatic, or functionalities, such as polar and non-polar.Other types of useful materials having cationically active functionalgroups include vinyl ethers, oxetanes, spiro-orthocarbonates,spiro-orthoesters, and the like.

[0027] The resin may be chosen from acrylate-based compositions thatcontain a free radically active functional group. Materials having freeradically active functional groups include monomers, oligomers, andpolymers having one or more ethylenically unsaturated groups. As usedherein, “free radically active functional group” is a chemical moietythat is activated in the presence of an initiator capable of initiatingfree radical polymerization such that it is available for reaction withother compounds bearing free radically active functional groups.Suitable materials contain at least one ethylenically unsaturated bond,and are capable of undergoing addition polymerization. Such freeradically polymerizable materials include mono-, di- or poly- acrylatesand methacrylates such as methyl acrylate, methyl methacrylate, ethylacrylate, isopropyl methacrylate, n-hexyl acrylate, stearyl acrylate,allyl acrylate, glycerol diacrylate, glycerol triacrylate,ethyleneglycol diacrylate, diethyleneglycol diacrylate.triethyleneglycol dimethacrylate, 1,3-propanediol diacrylate,1,3-propanediol dimethacrylate, trimethylolpropane triacrylate,1,2,4-butanetriol trimethacrylate, 1,4-cyclohexanediol diacrylate,pentaerythritol triacrylate, pentaerythritol tetraacrylate,pentaerythritol tetramethacrylate, sorbitol hexacrylate,bis[1-(2-acryloxy)]-p-ethoxyphenyldimethylmethane,bis[1-(3-acryloxy-2-hydroxy)]-p-propoxyphenyldimethylmethane, andtrihydroxyethyl-isocyanurate trimethacrylate; the bis-acrylates andbis-methacrylates of polyethylene glycols of molecular weight 200-500,copolymerizable mixtures of acrylated monomers such as those in U.S.Pat. No. 4,652,274, and acrylated oligomers such as those of U.S. Pat.No. 4,642,126; and vinyl compounds such as styrene, diallyl phthalate,divinyl succinate, divinyl adipate and divinylphthalate. Mixtures of twoor more of these free radically polymerizable materials can be used ifdesired.

[0028] If desired, both cationically active and free radically activefunctional groups may be contained in a single molecule. Such moleculesmay be obtained, for example, by reacting a di- or poly-epoxide with oneor more equivalents of an ethylenically unsaturated carboxylic acid. Anexample of such a material is the reaction product of UVR-6105(available from Union Carbide) with one equivalent of methacrylic acid.Commercially available materials having epoxy and free-radically activefunctionalities include the “Cyclomer” series, such as Cyclomer M-100,M-101, or A-200 available from Daicel Chemical, Japan, and Ebecryl-3605available from Radcure Specialties.

[0029] The resin can also include an acid functionality, such ascarboxylic acid, phosphoric and phosphonic acids. Examples of suchcompounds include the aliphatic carboxy compounds, such as acrylic acid,methacrylic acid, maleic acid, fumaric acid, itaconic acid, crotonicacid, aconitic acid, glutaconic acid, mesaconic, citraconic acid, acid,tiglicinic acid, 2-chloroacrylic acid, 3-chloroacrylic acid,2-bromoacrylic acid, 1-methacryloyl malonic acid, 1-acryloyl malic acid,N-methacryloyl and N-acryloyl derivatives of amino acids, and acids suchas tartaric acid, citric acid, malic acid that have been furtherfunctionalized with an ethylenic functionality. For example, citric acidmay be ethylenically functionalized by substituting with an acryloyl ormethacryloyl functionality. These polymerizable groups may be attacheddirectly to the acid containing compound, or may be optionally attachedthrough a linking group. Preferred linking groups include substituted orunsubstituted alkyl, alkoxyalkyl, aryl, aryloxyalkyl, alkoxyaryl,aralkyl or alkaryl groups. Particularly preferred linking groupscomprise an ester functionality and most particularly preferred linkinggroups comprise an amide functionality.

[0030] Polymeric initiator systems for the above resins would no longerbe limited to systems which are compatible with the oral environment asthe bulk of the polymerization of the resin constituents would occuroutside of the patient's mouth, such as in a manufacturing facilitywhere the mill blanks may be produced. Thus, many of the commonly knownpolymerization systems may be employed, such as curing systems involving2-part resins, heat, radiation, redox reactions or combinations thereof.By having the capability of employing various polymerization systems,waiting time for the patient is drastically reduced, as those particularsteps would be completed in the manufacturing site or laboratory.However, since a composite mill blank provides a practitioner theopportunity to finish a prosthetic at chairside (i.e while the patientwaits), it is preferred that polymeric initiator systems that arecompatible with the oral environment are employed.

[0031] One class of useful initiators includes sources of speciescapable of initiating both free radical and cationic polymerization.

[0032] Preferred free radical polymerization systems contain threecomponents: an onium salt, a sensitizer, and a free radical donor.Suitable salts include mixed ligand arene cyclopentadienyl metal saltswith complex metal halide ions, as described in “CRC Handbook of OrganicPhotochemistry”, vol II, ed. J. C. Scaiano, pp. 335-339 (1989).Preferably, the source is an onium salt such as a sulfonium or iodoniumsalt. Of the onium salts, iodonium salts (e.g., aryl iodonium salts) areparticularly useful. The iodonium salt should be soluble in thecomposition and preferably is shelf-stable, meaning it does notspontaneously promote polymerization when dissolved therein in thepresence of the cationic polymerization modifier and photosensitizer (ifincluded). Accordingly, selection of a particular iodonium salt maydepend to some extent upon the particular polymerizable reactants,cationic polymerization modifiers, and sensitizers (if included).

[0033] Suitable iodonium salts are described in U.S. Pat. Nos.3,729,313; 3,741,769; 4,250,053; 4,394,403; and 5,545,676, thedisclosures of which are incorporated herein by reference. The iodoniumsalt can be a simple salt, containing an anion such as Cl⁻, Br⁻, I⁻,C₄H₅SO₃ ⁻, or C(SO₂CF₃)₃ ⁻; or a metal complex salt containing anantimonate, arsenate, phosphate, or borate such as SbF₅OH⁻, AsF₆ ⁻, orB(C₆F₅)₄ ⁻. Mixtures of iodonium salts can be used if desired.

[0034] The initiation system may also include a sensitizer such as avisible light sensitizer that is soluble in the polymerizablecomposition. The sensitizer preferably is capable of absorbing lighthaving wavelengths in the range from about 300 to about 1000 nanometers.

[0035] Examples of suitable sensitizers include ketones, coumarin dyes(e.g., ketocoumarins), xanthene dyes, acridine dyes, thiazole dyes,thiazine dyes, oxazine dyes, azine dyes, aminoketone dyes, porphyrins,aromatic polycyclic hydrocarbons, p-substituted aminostyryl ketonecompounds, aminotriaryl methanes, merocyanines, squarylium dyes, andpyridinium dyes. Ketones (e.g., monoketones or alpha-diketones),ketocoumarins, aminoarylketones, and p-substituted aminostyryl ketonecompounds are preferred sensitizers. For applications requiring deepcure of epoxy-containing materials (e.g., cure of highly filledcomposites), it is preferred to employ sensitizers having an extinctioncoefficient below about 100 lmole⁻¹cm⁻¹, more preferably about or below100 lmole⁻¹cm⁻¹, at the desired wavelength of irradiation forphotopolymerization. The alpha-diketones are an example of a class ofsensitizers having this property, and are particularly preferred fordental applications.

[0036] Examples of particularly preferred visible light sensitizersinclude camphorquinione; glyoxal; biacetyl,3,3,6,6-tetramethylcyclohexanedione;3,3,7,7-tetramethyl-1,2-cycloheptanedione;3,3,8,8-tetramethyl-1,2-cyclooctanedione;3,3,18,18-tetramethyl-1,2-cyclooctadecanedione; dipivaloyl; benzil;furil; hydroxybenzil; 2,3-butanedione; 2,3-pentanedioone;2,3-hexanedione; 3,4-hexanedione; 2,3-heptanedione; 3,4-heptanedione;2,3-octanedione; 4,5-octanedione; and 1,2-cyclohexanedione; Of these,camphorquinone is the most preferred sensitizer.

[0037] The third component in the free radical polymerization system isthe electron donor. A wide variety of donors can be employed. The donoris soluble in the resin component of the mill blank processor and shouldmeet the oxidation potential (E_(o)x) limitation discussed in moredetail below. Preferably, the donor also is selected based in part uponshelf stability considerations. Accordingly, a selection of a particulardonor may depend in part on the resin component, iodonium salt andsensitizer chosen. Suitable donors are capable of increasing the speedof cure or depth of cure of a composition of the invention upon exposureto light of the desired wavelength. Also, the donor has an E_(o)xgreater than zero and less than or equal to E_(o)x (p-dimethoxybenzene).Preferably E_(o)x (donor) is between about 0.5 and 1 volts against asaturated calomel electrode. E_(o)x (donor) values can be measuredexperimentally, or obtained from references such as N. L. Weinburg, Ed.,Technique of Electroorganic Synthesis Part II Techniques of Chemistry,Vol. V (1975), and C. K. Mann and K. K. Barnes, ElectrochemicalReactions in Nonaqueous Systems (1970).

[0038] In the cases where cationic polymerization occurs, it may bedesirable to delay the onset of polymerization. For example, in the caseof a hybrid composition that includes both free radically activefunctional groups and cationically active functional groups, it may bedesirable to use an initiation system suitable for initiating both freeradical and cationic polymerization which is designed such that for agiven reaction temperature, photoinitiation of free radicalpolymerization occurs after a finite induction period T₁ andphotoinitiation of cationic polymerization occurs after a finiteinduction period T₃, where T₃ is greater than T₁. T₁ and T₃ are measuredrelative to administration of the first dose of actinic radiation whichoccurs at T₀. Such initiation systems are described in Oxman et al.,“Compositions Featuring Cationically Active and Free Radically ActiveFunctional Groups, and Methods for Polymerizing Such Compositions,”filed Jun. 5, 1998 and bearing U.S. Ser. No. 09/092,550, which isassigned to the same assignee as the present application and herebyincorporated by reference. As described therein, the photoinitiationsystem includes: (i) a source of species capable of initiating freeradical polymerization of the free radically active functional group andcationic polymerization of the cationically active functional group; and(ii) a cationic polymerization modifier. The amount and type of modifierare selected such that in the absence of the modifier, initiation ofcationic polymerization under the same irradiation conditions occurs atthe end of a finite induction period T₂ (also measured relative to T₀),where T₂ is less than T₃.

[0039] The induction periods (T₁, T₂, and T₃) can be measured usingdifferential scanning calorimetry. Following the first irradiation eventat T₀, the enthalpy of the reaction is measured as a function of time.Both initiation of free radical polymerization and initiation ofcationic polymerization result in an increase in enthalpy, observed as apair of separate peaks when data is charted on a graph. The time atwhich initiation occurs is taken to be the time at which the enthalpybegins to rise.

[0040] The cationic polymerization modifier preferably has aphotoinduced potential less than that of 3-dimethylaminobenzoic acid ina standard solution of 2.9×10⁻⁵ moles/g diphenyliodoniumhexafluoroantimonate and 1.5×10⁻⁵ moles/g camphorquinone in 2-butanone,measured according to the procedure described in the aforementionedOxman et al. application. In general, useful cationic polymerizationmodifiers are typically bases having pK_(b) values, measured in aqueoussolution, of less than 10. Examples of classes of suitable cationicpolymerization modifiers include aromatic amines, aliphatic amines,aliphatic amides, aliphatic ureas; aliphatic and aromatic phosphines,and salts of organic or inorganic acids (e.g.., salts of sulfinic acid).Specific examples include 4-(dimethylamino)phenylacetic acid,dimethylaminophenethanol, dihydroxy p-toluidine,N-(3,5-dimethylphenyl)-N,N-dimethanolamine, 2,4,6-pentamethylaniline,dimethylbenzylamine, N,N-dimethylacetamide, tetramethylurea,N-methyldiethanolamine, triethylamine, 2-(methylamino)ethanol,dibutylamine, diethanolamine, N-ethylmorpholine,trimethyl-1,3-propanediamine, 3-quinuclidinol, triphenylphosphine,sodium toluene sulfinate, tricyclohexylphosphine, N-methylpyrollidone,and t-butyldimethylaniline. These modifiers may be used alone or incombination with each other, or with a material having photoinducedpotential greater than that of 3-dimethylaminobenzoic acid in a standardsolution of 2.9×10⁻⁵ moles/g diphenyliodonium hexafluoroantimonate and1.5×10⁻⁵ moles/g camphorquinone in 2-butanone; an example of such amaterial is ethyl 4-(dimethylamino)benzoate (“EDMAB”).

[0041] In other cases, it may be desirable to accelerate initiation ofcationic polymerization. For example, in certain hybrid compositions itmay be desirable to achieve near-simultaneous initiation of the freeradically active functional groups and the cationically activefunctional groups. Examples of suitable initiation systems foraccomplishing this objective are described in Oxman et al., U.S. Ser.No. 08/838,835 filed Apr. 11, 1997 entitled “Ternary PhotoinitiatorSystem for Curing of Epoxy/Polyol Resin Compositions” and Oxman et al.,U.S. Ser. No. 08/840,093 filed Apr. 11, 1997 entitled “TernaryPhotoinitiator System for Curing of Epoxy Resins,” both of which areassigned to the same assignee as the present application and herebyincorporated by reference. As described therein, the photoinitiatorsystem includes an iodonium salt (e.g., an aryliodonium salt), a visiblelight sensitizer (e.g., camphorquinone), and an electron donor. Thesystems have a photoinduced potential greater than or equal to that of3-dimethylaminobenzoic acid in a standard solution of 2.9×10⁻⁵ moles/gdiphenyliodonium hexafluoroantimonate and 1.5×10⁻⁵ moles/gcamphorquinone in 2-butanone, measured according to the proceduredescribed in the aforementioned Oxman et al. applications. An example ofa suitable electron donor is ethyl 4-(dimethylamino)benzoate (“EDMAB”).

[0042] In the case of hybrid compositions that include both freeradically active functional groups and cationically active functionalgroups, it may be desirable to use one initiation system for freeradical polymerization and a separate initiation system for cationicpolymerization. The free radical polymerization initiation system isselected such that upon activation, only free radical polymerization isinitiated.

[0043] One class of initiators capable of initiating polymerization offree radically active functional groups, but not cationically activefunctional groups, includes conventional chemical initiator systems suchas a combination of a peroxide and an amine. These initiators, whichrely upon a thermal redox reaction, are often referred to as “auto-curecatalysts.” They are typically supplied as two-part systems in which thereactants are stored apart from each other and then combined immediatelyprior to use.

[0044] A second class of initiators capable of initiating polymerizationof free radically active functional groups, but not cationically activefunctional groups, includes free radical-generating photoinitiators,optionally combined with a photosensitizer or accelerator. Suchinitiators typically are capable of generating free radicals foraddition polymerization at some wavelength between 200 and 800 nm.Examples include alpha-diketones, monoketals of alpha-diketones orketoaldehydes, acyloins and their corresponding ethers,chromophore-substituted halomethyl-s-triazines, andchromophore-substituted halomethyl-oxadiazoles.

[0045] A third class of initiators capable of initiating polymerizationof free radically active functional groups, but not cationically activefunctional groups, includes free radical-generating thermal initiators.Examples include peroxides and azo compounds such asazobisisobutyronitrile (AIBN). A preferred thermal initiator is benzoylperoxide.

[0046] Dual initiation systems include a separate photoinitiation systemfor initiating polymerization of the cationically active functionalgroups. The cationic initiation system is selected such that activationof the free radical initiation system does not activate the cationicinitiation system. Examples of suitable cationic photoinitiation systemsfor a dual initiation system composition include the onium salts andmixed ligand arene cyclopentadienyl metal salts with complex metalhalide ions described above. Also suitable are cationic initiators thatare activated by heat, or part cationic initiators. Such systems aredescribed in “Chemistry and Technology of Epoxy Resins,” ed. by B.Ellis, Chapman & Hall, 1993.

[0047] A filler for the present invention is preferably a finely dividedmaterial that may optionally have an organic coating. Suitable coatingsinclude silane or encapsulation in a polymeric matrix.

[0048] Fillers may be selected from one or more of any material suitablefor incorporation in compositions used for medical applications, such asfillers currently used in dental restorative compositions and the like.The filler is finely divided and preferably has a maximum particlediameter less than about 50 micrometers and an average particle diameterless than about 10 micrometers. The filler can have a unimodal orpolymodal (e.g., bimodal) particle size distribution. The filler can bean inorganic material. It can also be a crosslinked organic materialthat is insoluble in the polymerizable resin, and is optionally filledwith inorganic filler. The filler should in any event be non-toxic andsuitable for use in the mouth. The filler can be radiopaque, radiolucentor non-radiopaque.

[0049] Examples of suitable inorganic fillers are naturally-occurring orsynthetic materials such as quartz, nitrides (e.g., silicon nitride);glasses containing, for example Ce, Sb, Sn, Zr, Sr, Ba, An, La, Y andAl; colloidal silica; feldspar; borosilicate glass; kaolin; talc;titania; and zinc glass; low Mohs hardness fillers such as thosedescribed in U.S. Pat. No. 4,695,251; and submicron silica particles(e.g., pyrogenic silicas such as the “Aerosil” Series “OX 50”, “130”,“150” and “200” silicas sold by Degussa and “Cab-O-Sil M5” silica soldby Cabot Corp.). Examples of suitable organic filler particles includefilled or unfilled pulverized polycarbonates, polyepoxides, polyaramid,and the like. Preferred filler particles are quartz, barium glass, andnon-vitreous microparticles of the type described in U.S. Pat. No.4,503,169. Metallic fillers may also be incorporated, such asparticulate metal filler made from a pure metal such as those of GroupsIVA, VA, VIA, VIIA, VIII, IB, or IIB, aluminum, indium, and thallium ofGroup IIIB, and tin and lead of Group IVB, or alloys thereof.Conventional dental amalgam alloy powders, typically mixtures of silver,tin, copper, and zinc, may also optionally be incorporated. Theparticulate metallic filler preferably has an average particle size ofabout 1 micron to about 100 microns, more preferably 1 micron to about50 microns. Mixtures of these fillers are also contemplated, as well ascombination fillers made from organic and inorganic materials.Fluoroaluminiosilicate glass fillers, either untreated or silanoltreated, are particularly preferred. These glasses have the addedbenefit of releasing fluoride at the site of dental work when placed inthe oral environment.

[0050] Optionally, the surface of the filler particles may be treatedwith a surface treatment such as a coupling agent in order to enhancethe bond between the filler and the polymerizable resin. The couplingagent may be functionalized with reactive curing groups, such asacrylates, methacrylates, epoxies, and the like. Examples of couplingagents include gamma-methacryloxypropyltrimethloxysilane,gamma-mercaptopropyltriethoxysilane, gamma-aminopropyltrimethoxysilane,beta-(3,4-epoxycyclohexyl)ethyl-trimethoxysilane,gamma-glycidoxypropyltrimethoxysilane, and the like.

[0051] Preferable fillers are those that have been derived throughsol-gel processes. It has been surprisingly found that sol-gel derivedfillers impart superior machining characteristics to composites used fordental mill blanks. Moreover, it was surprisingly found that sol-gelderived fillers may be incorporated into resins at higher levels thanconventional milled glass fillers. Sol-gel processes for making fillersare described, for example, in U.S. Pat. No. 4,503,169 (Randklev) and byNoritake et al. in GB Patent 2291053 B. As used herein, “sol-gel” refersto any method of synthesizing inorganic particles that comprises a stepwherein at least one of the precursors is an aqueous or organicdispersion, sol, or solution.

[0052] A preferred method for preparing the sol-gel derivedmicroparticles or fillers for the present invention involves thecombining of (1) an aqueous or organic dispersion or sol of amorphoussilica with (2) an aqueous or organic dispersion, sol, or solution ofthe desired radiopacifying ceramic metal oxide or a precursor organic orinorganic compound which is calcinable to the desired radiopacifyingceramic metal oxide. For brevity, the aforementioned dispersion or solof silica will be sometimes referred to hereafter as the “silicastarting material”, and the aforementioned dispersion, sol, or solutionof the radiopacifying ceramic metal oxide or precursor compound willsometimes be referred to hereafter as the “ceramic metal oxide startingmaterial”. The mixture of silica starting material and ceramic metaloxide starting material is dried to a solid and fired to formmicroparticles. Comminution may optionally be done at any stage. Themicroparticles can then be combined with an appropriate resin to form acomposite of the invention.

[0053] Although either aqueous or organic silica starting materials canbe employed in the sol-gel method just described, aqueous silicastarting materials are preferred for reasons of economy. Suitableaqueous silica starting materials preferably contain colloidal silica atconcentrations of about 1 to 50 weight percent, more preferably 15 to 35weight percent. Suitable organic silica starting materials includeorgano-sols containing colloidal dispersions of silica in organicsolvents (preferably water-miscible polar organic solvents) such asethanol, normal or isopropyl alcohol, ethylene glycol, dimethylformamideand the various “Cellosolve” glycol ethers. The size of the colloidalsilica particles in the silica starting material can vary, e.g., from0.001 to 0.1 micrometers, preferably about 0.002 to 0.05 micrometers.Preferred sol-gel filters are those comprising zirconia and silica.

[0054] Another class of useful fillers are bioactive glasses andceramics. Examples include Bioglass™ (U.S. Biomaterials; Alachua, Fla.);Bio-Gran™ (Orthovita; Malvern, Pa.); Cerabone A-W (Nippon ElectricGlass: Japan); glasses comprising calcium oxide, silicon oxide, andphosphorous oxide; and the various phases of calcium phosphate includinghydroxyapatite, monetite, brushite, and whitlockite.

[0055] Optionally, dental mill blanks may contain fluoride-releasingagents. The benefits of fluoride in reducing the incidence of caries arewell established. Thus fluoride released from dental prostheses would beadvantageous. Fillers that impart fluoride release include ZnF₂, YbF₂,rare-earth fluorides, SnF₂, SnF₄, ZrF₄, NaF, CaF₂, YF₃, andfluoroaluminosilicate glasses. Rare earths are the elements of atomicweights 57-71, inclusive.

[0056] The fluoride-releasing material of the present invention may benaturally occuring or synthetic fluoride minerals, fluoride glass suchas fluoroaluminosilicate glass, simple and complex inorganic fluoridesalts, simple and complex organic fluoride salts or combinationsthereof. Optionally these fluoride sources can be treated with surfacetreatment agents.

[0057] Examples of the fluoride-releasing material arefluoroaluminosilicate glasses described in U.S. Pat. No. 4,3814,717,which may be optionally treated as described in U.S. Pat. No. 5,332,429,the disclosures of which are both incorporated by reference herein.

[0058] The fluoride releasing material may optionally be a metal complexdescribed by formula

M(G)_(g)(F)_(n) or M(G)_(g)(ZF_(m))_(n)

[0059] where M represents an element capable of forming a cationicspecies and having a valency of 2 or more,

[0060] G is an organic chelating moiety capable of complexing with theelement M,

[0061] Z is hydrogen, boron, nitrogen, phosphorus, sulfur, antimony,arsenic,

[0062] F is a fluoride atom, and

[0063] g, m and n are at least 1.

[0064] Examples of preferred M elements are the metals of groups IIA,IIIA, IVA, and transition and inner transition metal elements of theperiodic table. Specific examples include Ca⁺², Mg⁺², Sr⁺², Zn⁺², Al⁺³,Zr⁺⁴, Sn⁺², Yb⁺³, Y⁺³, Sn⁺. Most preferably, M is Zn⁺².

[0065] Compositions of the present invention may optionally comprise atleast two sources of fluoride. The first source is thefluoride-containing metal complex as described above. The second sourceis a fluoride-releasing fluoroaluminosilicate glass. With the use ofboth materials, excellent fluoride release is provided both in theinitial period and over the long term use of the composition.

[0066] The mill blanks of the present invention may optionally compriseadditional adjuvants suitable for use in the oral environment, includingcolorants, flavorants, anti-microbials, fragrance, stabilizers, andviscosity modifiers. Other suitable adjuvants include agents that impartfluorescence and/or opalescence.

[0067] As the polymer resin is initially a paste, any of the standardmethods for compounding paste may be used to form the compositematerial. Preferably, methods which optimize mixing and minimize theincidence of material discontinuities such as voids and cracks should beinstituted. For example, application of vacuum or pressure can bebeneficial during any stage of compounding, forming or curing the paste.Pressure can be applied by various means, including isostatic, uniaxial,centrifugal, impact, or pressurized gas. Heat may optionally be appliedat any stage. However, during curing, a uniform temperature in thesample is preferably maintained to minimize internal stresses.

[0068] During compounding and extrusion, methods that minimize andpreferably eliminate material discontinuities such as voids or bubblesare preferred. Preferably the blanks of the present invention aresubstantially free of discontinuities in the material that are largerthan about 1 millimeter. More preferably, fabrication techniques areemployed such that the material is substantially free of discontinuitiesin the material that are larger than about 0.1 millimeter. Mostpreferably, blanks of the present invention are substantially free ofdiscontinuities in the material that are larger than about 0.01millimeter.

[0069] Blanks of composite may be made in any desired shape or size,including cylinders, bars, cubes, polyhedra, ovoids, and plates. Moldsmay be made of a variety of materials, including stainless steel, cobaltalloys, nickel alloys, aluminum alloys, plastic, glass, ceramic, orcombinations thereof. Alternatively, a variety of methods for formingand shaping the blanks into any desired configuration can be employed,such as injection molding, centrifugal casting and extrusion. Duringpolymerization and curing, compression from springs or other means mayoptionally be used to reduce internal stresses. Preferably, the outersurface of the blank is smooth and non-tacky.

[0070] Curing may be performed in one or multiple stage methods. In atwo-stage process, it is preferred that initial curing provide amaterial sufficient to sustain the forces of milling or carving. Thesecond curing stage, therefore, can be performed on the composite aftera prosthetic is milled from a blank.

[0071] Cured blocks may be attached to mounting stubs to facilitateaffixation of the blank in a milling machine. Mounting stubs function ashandles from which a blank is held by as it is milled by a machine.

[0072] Various means of milling the mill blanks of the present inventionmay be employed to create custom-fit dental prosthetics having a desiredshape and morphology. The term “milling” as used herein means abrading,polishing, controlled vaporization, electronic discharge milling (EDM),cutting by water jet or laser or any other method of cutting, removing,shaping or carving material. While milling the blank by hand using ahand-held tool or instrument is possible, preferably the prosthetic ismilled by machine, including computer controlled milling equipment.However, a preferred device to create a prosthetic and achieve the fullbenefits of the composite material of the present invention is to use aCAD/CAM device capable of milling a blank, such as the Sirona Cerec 2machine. By using a CAD/CAM milling device, the prosthetic can befabricated efficiently and with precision. During milling, the contactarea may be dry, or it may be flushed with a lubricant. Alternatively,it may be flushed with an air or gas stream. Suitable lubricants arewell known in the art, and include water, oils, glycerine, ethyleneglycols, and silicones. After machine milling, some degree of finishing,polishing and adjustment may be necessary to obtain a custom fit in tothe mouth and/or aesthetic appearance.

[0073] A milled dental prosthetic can be attached to the tooth or bonestructure with conventional cements or adhesives or other appropriatemeans such as glass ionomer, resin cement, zinc phosphate, zincpolycarboxylate, compomer, or resin-modified glass. In addition,material can optionally be added to the milled prosthetic for variouspurposes including repair, correction, or enhancing esthetics. Theadditional material may be of one or more different shades or colors.The added material may be composite, ceramic, or metal. A light-curedcomposite is preferred.

[0074] To fabricate blanks of the present invention, the following stepsare preferably performed: Compound the paste; extrude the paste into amold; cure the paste via heat, light, microwave, e-beam or chemicalcure; remove the blank from its mold and trim excess if necessary; andoptionally, mount on a holder stub if necessary. A preferred method ofmaking the dental mill blank of the present invention comprises thesteps of a) mixing a paste comprising a resin and a filler, b) shapingthe paste into a desired configuration, c) minimizing materialdiscontinuities from the paste, d) curing the paste into a blank, and e)relieving internal stresses in the blank.

[0075] Optionally, where a mold is used to shape the paste, excess pastematerial can be trimmed from the mold. The cured past is then removedfrom the mold. Another optional step that can be performed in making amill blank is to mount a handle onto the cured paste. Preferably, thehandle is a holder stub.

[0076] Mill blanks of the present invention may be cured in a mannersuch that the material contains minimal internal stresses. This may beaccomplished, for example, by application of pressure on the compositematerial during the curing process. In the alternative, the avoidance ofinternal stress imparted by shrinkage may be obtained by selection ofmill blank components such that the overall composition exhibits littleor no shrinkage during cure. A preferred curing method entails the useof light to fast cure the composite. During this fast cure, thetemperature may optionally be adjusted and controlled. The fast curetechnique requires a subsequent heat treatment to effectuate stressrelief. Heat treatment of a cured blank requires the blank be heated fora sufficient time and at a sufficient temperature to effectivelyeliminate internal stresses such that the blank passes a Thermal ShockTest. Preferably, the blank is raised to a temperature of at or above Tg(glass transition temperature) of the resin component of the blank. Morepreferably, the blank is heated to above Tg and is maintained at thattemperature for at least about one-half hour.

[0077] A preferred method of heat treatment for a cured blank is toplace the blank in an oven and raise the oven temperature to about theTg of the resin component of the blank at a rate of about, for example,3-5° C. minute. Upon completing heat treatment, the blank is allowed toequilibrate to room temperature either by immersion into roomtemperature water or by slowly cooling via ambient temperature.Alternatively, the heat treatment may be accomplished by placing theblank in a preheated oven and maintaining the oven temperature at orabove Tg for a sufficient time to eliminate internal stresses in thecomposite blank.

[0078] Another method of curing the blanks of the present invention isthrough a slow cure using low intensity light. In this technique, cureis accomplished over a long period of time to minimize internalstresses, such that the resulting cured blank will pass a Thermal ShockTest. Preferably, the cure takes place over a time period of about 24hours, however it is envisioned that with proper equipment andprocedure, curing times may be shorter. Progress of this cure may beevaluated by ascertaining a sample of the material at predeterminedtimes over the cure time and evaluating progress of cure by BarcolHardness measurement.

[0079] Other techniques may be used to relieve the stress of mill blanksof the present invention, including application of energy in a formother than heat, such as sonic or microwave energy.

[0080] A preferred method for testing the existence of residual internalstress of a composite mill blank is the Thermal Shock Test involving theuse of liquid Nitrogen. Residual internal stress is undesirable becauseit adversely affects the structural integrity of the blank and increasesthe likelihood of later catastrophic failure of the blank or theultimate prosthetic. To conduct such a test, commercially availableliquid nitrogen is poured into a 250 milliliter (mL) Dewar flask. Afully cured mill blank is immersed in the liquid nitrogen untilexcessive bubbling subsides. If the blank explodes or experiences alarge crack while immersed in the liquid nitrogen, the blank fails theTest. If the blank does not explode or did not appear to have asubstantial crack, the mill blank must then be inspected for internalstress fractures (cracks). As used herein, a “crack” is defined asfissure where material has separated or broken away.

[0081] To inspect for cracks from internal stresses, the mill blankshould be removed from the flask and brought to room temperature. Thismay be done slowly by immersing the blank in room temperature water. Theblank can then be dried off and inspected for cracking. If, after up toabout one hour upon the blank returning to room temperature, the blankcracks, this result also indicates a failing score for the Test.

[0082] It is essential for proper test results that the test material befree of any gross interphase between two or more materials. Thus, if amill blank is attached to a stub, the mounting stub must be removedprior to immersing the blank in the liquid nitrogen-filled flask.Similarly, if a mill blank comprises more than one piece of material,whether it is of the same or different composition as the test material,then the material that will not ultimately be milled into a prostheticmust be removed prior to thermal shock testing.

[0083] Inspection may first be done with an unaided human eye, lookingspecifically for cracks that may have propagated to the blank's surface.However, while visual inspection is useful for observing cracks anddiscontinuities at or near the surface, it is desirable to have anondestructive method for detecting these defects throughout the entiresample. Thus, further inspection is preferably conducted using an x-raydevice that can reveal internal cracks and discontinuities. Inspectionmay be alternatively performed by other methods known in the art, suchas ultrasonic imaging, CAT scans, NMR imaging, or eddy currentmeasurements.

[0084] X-ray radiography is preferably used to detect cracks anddiscontinuities less than about 1 mm in size. This method can be used tomeasure the incidence of cracks and discontinuities in a blank or abatch of blanks, and furthermore as a tool for optimizing thefabrication process to minimize the incidence of cracks anddiscontinuities. This method is particularly useful as a quality test,wherein blanks that have detectable cracks or discontinuities aredisqualified for use.

[0085] X-ray radiography comprises exposing the block to x-rays whilesimultaneously recording them opposite the source. Methods, materials,and equipment for such radiography are well known in the medical art.The x-ray energy and exposure times are appropriately adjusted to thematerial and geometries of the blanks to be inspected.

[0086] The following examples are meant to be illustrative of theinvention and should not meant to limit the scope or range of theinvention. Unless otherwise indicated, all parts and percentages are byweight, and all molecular weights are weight average molecular weights.

[0087] TEST METHODS

[0088] The following methods were used to evaluate the examples andsamples.

[0089] Thermal Shock Test: Liquid Nitrogen Dip Test

[0090] A 250 mL Dewar flask (Pope Scientific, ™8600) was filled with 200mL, of industrial grade liquid nitrogen. Samples (composite mill blanks)were immersed in the liquid nitrogen until excessive bubbling subsided(approximately two minutes). The blanks were removed from the liquidnitrogen and allowed to equilibrate to room temperature by immersing theblanks in room temperature water. The samples were dried off andvisually inspected for cracks.

[0091] In the case of certain materials that are peculiarly sensitive tothe Thermal Shock Test, special sample handling procedures may berequired to assure appropriate evaluation of internal stress as comparedto other factors. For example, some mill blank materials may byhydrophilic to the point of taking up atmospheric water during thecooling process of the heat treatment. The presence of such atmosphericwater, particularly in a non-uniform concentration throughout the millblank, may result in test failure even though the sample does notpossess internal stress imparted by polymerization shrinkage.Maintenance of such samples in a desiccated environment (e.g. during thecooling step of the heat treatment) before the Thermal Shock Test willassure that an otherwise acceptable mill blank does not show a falsefailure of the Thermal Shock test. Alternative evaluation techniques maybe required to show that certain materials are sufficiently free ofinternal stress so that they would pass the Thermal Shock Test absentthe peculiarity of the materials that makes such passage impossible.

[0092] Barcol Hardness

[0093] Hardness of a cured sample was measured using a “Barber ColemanImpressor” Model GYZJ 934-1 (Barber Coleman; Rockford, Ill.).

[0094] Cuttability Value (used for evaluating Samples 1-10)

[0095] A Unitek™ electrical handpiece (model No.738-151, 3M Unitek,Monrovia, Calif.) was clamped at its base such that it was level andpivoted freely about its base. Guides were placed to prevent sidewaysmotion of the handpiece. A 151.8 g weight was suspended from the neck ofthe handpiece 10 centimeters (cm) from the base. The diamond rested on amill blank secured to a platform; the cutting tool was 17.5 cm from thehandpiece base.

[0096] A CEREC™ cylinder diamond 1.6 millimeters (mm) in diameter(Sirona Dental Systems; Bensheim, Germany) was secured in the handpiece.The length of contact between the diamond and the sample was 5 mm. This5 mm diamond segment was allowed to rest on the block. The handpiece wasoperated at its top speed (approximately 20,000 rpm) for 60 seconds ±1second. The diamond and work area was flushed continuously withdeionized water. At least three cuts were made on each block. A STARRETT721 Electronic Digital Caliper (L. S. Starrett Co.; Athol, Mass.) wasused to measure the height of the block adjacent to each cut and thedistance from the botttom of the cut to the opposite edge of the block.The depth of the cut was calculated from the difference of these twomeasurements. A new diamond was used to test each block.

[0097] X-Ray Inspection

[0098] X-ray radiography was performed on a Profexray(TM) Rocket 300X-ray unit (Litton Industries, Des Plaines, Ill.). 3M Diagnostic ImagingFilm, Ultra Detail Plus, Rare Earth Veterinary X-ray type (3M, St. Paul,Minn.) was used to record the x-ray image; the film was developed with a3M XT 2000 Film Processor (3M, St. Paul, Minn.). The samples were setdirectly on the film container, resulting in a 1:1 magnification.Settings of 300 mA, 80 kV were used; images were taken at variousexposure times.

[0099] The resulting radiographs were viewed on a x-ray illuminatorunit, and examined for the presence of any cracks or discontinuities,e.g. voids, pores, or knit lines.

EXAMPLES

[0100] Preparatory Example 1

[0101] A light curable resin was compounded by dissolving and mixing thefollowing constituents:

[0102] 0.01 pbw Ethyl 4-dimethylaminobenzoate (EDMAB)

[0103] 0.0017 pbw camphorquinonie (CPQ)

[0104] 0.01 pbw 2-(2′-Hydroxy-5′-methylphenyl)Benzotriazole(“Tinuvin-P”; Ciba-Geigy Corp.; Hawthorne, N.Y.)

[0105] 0.006 pbw Diphenyl Iodonium Hexafluorophosphate

[0106] 0.4862 pbw2,2-bis[4-(2-Hydroxy-3-methacryloyloxy-propoxy)phenyl]propane (Bis-GMA)

[0107] 0.4862 pbw triethyleneglycol dimethacrylate (TEGDMA)

Preparatory Example 2

[0108] A sol-gel derived filler was prepared as follows: 25.5 partssilica sol (“Ludox” LS:E.I duPont de Nemours & Co.) were acidified bythe rapid addition of 0.255 parts concentrated nitric acid. In aseparate vessel, 12.9 parts ion-exchanged zirconyl acetate (MagnesiumElektron, Inc.) were diluted with 20 parts deionized water and theresultant solution acidified with 0.255 parts concentrated nitric acid.The silica sol was pumped into the stirred zirconyl acetate solution andmixed for one hour. The stirred mixture was filtered through a 3micrometer filter followed by a 1 micrometer filter. The filtrate waspoured into trays to a depth of about 25 mm and dried at 65° C. in aforced air oven for about 35 hours (hrs). The resultant dried materialwas removed from the oven and tumbled through a rotary tube furnace(Harper Furnace Corp.), which was preheated to 950° C. The calcinedmaterial was comminuted in a tumbling ball mill with ¼″ alumina mediauntil an average particle size of 0.5-1.2 micrometers (as measured on aMicromeritics 5100 sedigraph) was achieved. The mill charge included 75parts calcined material, 3 parts methanol, 1.9 parts benzoic acid, and1.1 parts deionized water. The filler was then loaded into ceramicsaggers and fired in an electric furnace (L&L Furnace Corp.) in air at880-900° C. for approximately 8 hrs. The fired filler was thenball-milled for 4-5 hrs. The mill charge included 32 parts fired filler,1.25 parts ethanol, and 0.3 parts deionized water. Next, the filler waspassed through a 74 micrometer nylon screen in a vibratory screener(Vortisiv V/S 10010). The filler was then blended in a V-blender(Patterson-Kelly Corp.) for about 15 min.

[0109] Silane treatment was as follows: 32 parts by weight (pbw) of thefiller was added to 48.94 pbw of deionized water under vigorousstirring. Trifluoroacetic acid (TFAA), 0.104 pbw, was added slowly. ThepH was then adjusted to 3.0-3.3. by adding further 5 pbw increments ofTFAA. Then, 3.56 pbw of silane A-174 (Union Carbide; Stamford, Conn.)was added. After stirring vigorously for 2 hrs a solution of 0.0957 pbwof calcium hydroxide and 0.30 pbw of deionized water was added andstirred an additional 5 minutes. The slurry was poured into a tray linedwith a plastic sheet, and then dried in an oven set at 90° C. for 13hours. The cakes of dried filler were crushed and passed through a 74 μmscreen.

Preparatory Example 3

[0110] A commerical barium glass with a nominal average particle size of0.7 μm (type 8235, grade UF-0.7 (Schott Glaswerke; Landshut Germany) wassilane treated as follows: 2000 pbw of the glass was added to 3242 pbwof deionized water under vigorous stirring. 6.5 pbw of Trifluoroaceticacid (TFAA) was added slowly and the pH was then adjusted to 3.0-3.3. byadding further 5 pbw increments of TFAA. Then, 40.0 pbw of silane A-174(Witco; Greenwich, Conn.) was added. After stirring vigorously for 2hours, a solution of 5.98 pbw of calcium hydroxide and 200 g ofdeionized water was added and stirred an additional 5 minutes. Theslurry was poured into a tray lined with a plastic sheet, and then driedin an oven set at 90° C. for 13 hours. The cakes of dried filler werecrushed and passed through a 74 μm screen. The vendor literature shows acoefficient of thermal expansion (CTE) of 4.7×10⁻⁶/° C., refractiveindex of 155.1, density of 3.04 g/cc, and a nominal composition of 30%BaO, 10% B₂O₃, 10% Al₂O₃, and 50% SiO₂ by weight.

Preparatory Example 4

[0111] Fumed silica, Aerosil OX50 (Degussa AG; Frankfurt, Germany), wassilane treated as follows: A-174 (3.7 g) was added with stirring to 50 gof deionized water acidified to pH 3-3.3 by dropwise addition oftrifluoroacetic acid. The resultant mixture was stirred at about 25° C.for 1 hour at which time 95 g of OX-50 were added to the mixture withcontinued stirring for 4 hours. The slurry was poured into aplastic-lined tray and dried at 35° C. for 36 hours. The silanol treateddried powder was sieved through a 74 micrometer mesh screen.

Preparatory Example 5

[0112] Silane treated quartz was prepared as follows. Quartz rock washeated to about 660° C., quenched in water, drained, then dried in aforced air oven for 16 hours at about 200° F. The quenched quartz wascombined with quartz media into a mill and tumbled for about 70 hours.The charge included 99 pbw quenched quartz and 1 part methanol. Theresulting particles were blended with 0.1 wt. % carbon black in aV-blender for 1 hour, then fired in an electric furnace at about 950° C.for 4 hours. The resulting particles were then passed through a 100micrometer nylon screen, and blended in a V-blender for 30 minutes.34.68 pbw of deionized water was adjusted to ph of 3.00-3.30 with about0.1 pbw of TFAA. A-174 silane, 1.74 pbw, was added and then vigorouslystirred for 1 hour. The quartz powder and Aerosil R972 fumed SiO₂(Degussa), 62.43 and 1.01 pbw, respectively, were slowly charged to thevessel. After 90 minutes of stirring, the slurry was dried in tray at60° C. for 18 hours and then sieved through a 70 μm screen.

[0113] Curing and Heat Treatment Samples

[0114] Paste Samples A-I

[0115] A cartridge of composite material containing 500 g of Sample 9was placed in an air oven (“Stabil-Therm”; Blue-M Electric Co.) at 60°C. for 2 hours. Clean glass tubes, marked to fill height and plugged atthe bottom end with silicone plugs, were placed in the oven at 60° C.for 1 hour.

[0116] The glass tubes were filled with the composite to the fill lineand returned to the air oven for 30 minutes. The filled tubes werecentrifuged (International Eqpt. Co.) at 2850 rpm for 60 minutes.

[0117] Fast Cure

[0118] Centrifuged paste contained in glass tubes were placed in an 800mL beaker containing about 400 mL of room temperature water. The tubeswere placed in the beaker evenly spaced apart, with the silicone plug atthe bottom. The beaker was then placed in a Suntest Box (SuntestAccelerated Exposure Table Unit #7011, Germany) for 10 minutes. Aftercuring, the tubes were removed from the beaker and the silicone plugswere removed. The tubes were then inverted from their original curingposition and replaced in the beaker for an additional 10 minutes ofcuring inside the Suntest Box. The tubes were then removed from theSuntest Box and the glass tubes were separated from the cured compositeblank. One blank was cut in half and inspected for discontinuties andcracks.

[0119] Slow Cure

[0120] The glass tubes containing centrifuged paste were set on aGlow-Box (Model 12.12D, 22 Watts power consumption—available from 12RCo., Cheltenham, Pa.) for 24 hours with the silicone plugs at the top.The Glow Box provided approximately 300 foot candles of light output(measured by GE Light Meter Type 213; Cleveland, Ohio). The siliconeplugs were then removed. The tubes were inverted from their originalcuring position and replaced on the Glow Box for an additional 24 hoursof curing. The tubes were then removed from the Glow Box and the glasstubes were separated from the cured composite blank. One blank was cutin half and inspected for discontinuities and cracks. Barcol hardnessmeasurements were taken.

[0121] Post Cure

[0122] Blanks cured by both the slow and fast light cure methods abovewere then post-cured in a Suntest Box for 10 minutes.

[0123] Heat Treatment

[0124] Fast light cured blanks were placed in a forced air oven(“Stabil-Therm,” Blue-M Electric Co.). The oven was ramped up to 100° C.at 4° C./minute. The oven temperature was maintained for 30 minutes. Theoven was then shut off and the blanks were permitted to equilibrate toambient temperature before they were tested.

[0125] Samples A through D were cured on the Glow-Box for the timesshown in Table 1. Approximately 3 mm were cut off from each end. Thesamples were sectioned with a diamond saw into equidistant sections ofapproximately 10 mm thickness to produce 5 interfaces. The finaldimension of each section was 14 mm×10 mm. Barcol hardness measurementswith a GYZJ 934-1 hardness meter were taken in the center of eachsection on the obverse side of the section to the Glow-Box. An averageof over three measurements were recorded.

[0126] A similar procedure for the samples made using the fast curemethod (Samples E-I) was followed. Data is shown in Table 2. TABLE 1Slow Cure Process Barcol Hardness Sample Cure Time (hrs.) 1 2 3 4 5 A 2440 20  0  0  0 B 48 82 80 73 63  0 C 72 87 84 81 80 77 D 96 88 88 87 8684

[0127] TABLE 2 Fast Cure Process Barcol Hardness Sample Cure Time (min.)1 2 3 4 5 E  5 0 0 0 0 0 F 10 0 0 0 0 0 G 15 48 51 50 52 55 H 20 84 8581 84 84 I 25 88 90 91 89 91

[0128] Forty-one samples were made using the same procedure describedabove for making Samples A-I. Eight samples were slow cured, twelve werefast cured, and the remaining twenty-one samples were fast cured andheat-treated. All forty-one samples tested using the Thermal Shock Test.TABLE 3 Results of Thermal Shock Test Cure Mode Heat Treat Pass FailSlow No 8 0 Fast No 0 12 Fast Yes 21 0

[0129] Sample Preparation

[0130] Composite Paste Samples 1-8 were prepared by charging fillers andresin to a plastic beaker and then stirring and kneading theseconstituents into a paste with a flattened glass rod. TABLE 4 SampleAmount of Preparatory Type and Amount of Filler No. Example 1 Resin(pbw) (pbw) 1 30 70, Preparatory Example 2 2 30 70, Preparatory Example3 3 30 70, Preparatory Example 5 4 40 60, Preparatory Example 4 5 20 80,Preparatory Example 2 6 20 80, Preparatory Example 5 7 40 60,Preparatory Example 3 8 50 50, Preparatory Example 4  9* 14.7 85.3Preparatory Example 2

[0131] * Sample No. 9 was compounded in a double planetary mixer.

[0132] A Comparative Sample 10 was made from commerically available VitaMark II A3C/I12 Restorative (Vita Zahnfabrik, Bad Sackingen, Germany).When possible, pastes were compounded in a range containing filler from70 to 80 weight percent. With the Preparatory Example 3 filler, Schott8235 Glass, the paste became dry and crumbly at about 73-76% by weightof filler. With the Preparatory Example 4 fillers Aerosil OX50, thepaste became far too thick to mix by hand when the filler content wasgreater than about 60% by weight.

[0133] Loading Curing and Heat Treatment of Samples 1-9

[0134] The paste was filled into plastic cuvets and then compressedmanually with a stainless steel plunger. The filled cuvets were thenplaced in a Kulzer™ Dentacolor™ XS Curing Unit™ (Heraueus Kulzer;Irvine, Calif.) and cured for 90 seconds on each long side. Total curingtime was 360 seconds. The plastic cuvet was then broken off to produce acured mill block of approximately 10×10 mm cross section by 3-4 cm long.Blocks were heat treated in an oven by placing them in a cool oven. Theoven was then heated to 100 C and maintained at that temperature for onehour. The oven was then turned off and the samples were allowed to coolin the oven to room temperature.

[0135] Each sample was evaluated for cuttability and Barcol Hardness.Barcol Hardness of the composite blanks was tested with a Barber ColemanImpressor Model GYZJ 934-1 (Barber Coleman; Rockford, Ill.). An averageof the three readings was recorded.

[0136] Cuttability is calculated by the following equation, percentincrease compared to Sample 8 equals [(Cuttability—Cuttability of Sample8)/Cuttability of Sample 8] multiplied by 100. TABLE 5 % IncreaseCuttability: of Cuttability Filler or Filler Avg Compared to Sample No.Product wt % Depth (mm) Sample 8 Barcol-avg 1 Sol-gel 70 0.93 70 79.3 2Glass 70 0.71 29 86.0 3 Quartz 70 0.72 32 77.3 4 Fumed 60 0.56 2 78.3Silica 5 Sol-gel 80 1.24 127 85.0 6 Quartz 80 1.45 166 80.3 7 Glass 601.05 93 75.5 8 Fumed 50 0.55 0 75.0 Silica 9 Sol-Gel 85.3 2.01 268 89.5Comparative Vita Mark II 0.83 44 — 10  A3C/I12 (no heat treatment)

[0137] Sample 11

[0138] 3M F2000 shade A2 (3M Co.; St. Paul, Minn.), fluoride-releasingmaterial, was extruded into a cuvet to about ¾ full. The filled cuvetwas placed standing vertically in a Hanau Sun-Test box with a xenon lampand exposed to light for 30 min. The cuvet was rotated lengthwise andexposed to light another 30 min. The cured block was heat treated in aDespatch oven at 100° C./60 min., then allowed to cool in the oven.

[0139] X-Ray Analysis of Samples

[0140] Examples X1-X8 were fabricated in the same way as Samples E-Iexcept that they were centrifuged at 2700 RPM, and light cured for 30minutes immersed in water; and not heat-treated.

[0141] Examples X9-X12 were fabricated in the same way as Sample E-Iexcept that they were centrifuged at 2700 RPM, and light cured for 41minutes immersed in water; and heat-treated in the same way as samples1-9.

[0142] Examples X13-X22 were fabricated in the same way as Samples E-Iexcept that they were centrifuged at 2700 RPM, and light cured for 30minutes immersed in water; and heat-treated in the same way as samples1-9.

[0143] Example X23 was fabricated in the same way as Samples E-I exceptthat it was centrifuged at 2400 RPM, and light cured for 30 minutesimmersed in water; and heat-treated in the same way as samples 1-9.

[0144] Examples X24-28 are commercial Vita Mark II Vitablocs.

[0145] Examples X29-X32 were fabricated in the same way as Samples A-Dexcept that the paste was heated to 45° C. for filling. TABLE 6 ExposureSample # time (sec) Observation  X1 1/30 many pores, ˜0.5-2 mm  X2 1/30no cracks or other discontinuities visible  X3 1/30 no cracks or otherdiscontinuities visible  X4 1/30 no cracks or other discontinuitiesvisible  X5 1/30 several pores 1-4 mm  X6 1/30 no cracks or otherdiscontinuities visible  X7 1/30 no cracks or other discontinuitiesvisible  X8 1/30 no cracks or other discontinuities visible  X9 1/30 nocracks or other discontinuities visible X10 1/30 no cracks or otherdiscontinuities visible X11 1/30 large pit at end open to surface X121/30 large pit at end open to surface X13 1/30 flat pores, about 0.1 mmthick × 3 mm long X14 1/30 flat pores, about 0.1 mm thick × 3 mm longX15 1/30 flat pores, about 0.1 mm thick × 3 mm long X16 1/30 flat pores,about 0.1 mm thick × 3 mm long X17 1/30 no cracks or otherdiscontinuities visible X18 1/30 no cracks or other discontinuitiesvisible X19 1/30 no cracks or other discontinuities visible X20 1/30 nocracks or other discontinuities visible X21 1/30 flat pores, about 0.1mm thick × 3 mm long X22 1/30 flat pores, about 0.1 mm thick × 3 mm longX23 1/30 one pore ˜3 mm; one crack ˜5 mm long X24 1/30 no cracks orother discontinuities visible X25 1/30 no cracks or otherdiscontinuities visible X26 1/30 no cracks or other discontinuitiesvisible X27 1/30 no cracks or other discontinuities visible X28 1/30 nocracks or other discontinuities visible X29 1/30 no cracks or otherdiscontinuities visible X30 1/30 narrow longitudinal crack 0.1 mm widetop to bottom X31 1/30 small crack ˜0.1 mm wide X32 1/30 small crack<0.1 mm wide

We claim:
 1. A carvable mill blank for making a dental prostheticcomprising a) a polymeric resin and b) a filler, wherein the blank issubstantially free of cracks and fabricated such that the blank passes aThermal Shock Test.
 2. The blank of claim 1 wherein the blank issubstantially free of discontinuities in the material that are largerthan about 1 millimeter.
 3. The blank of claim 1 wherein the blank issubstantially free of discontinuities in the material that are largerthan about 0.1 millimeter.
 4. The blank of claim 1 wherein the blank issubstantially free of discontinuities in the material that are largerthan about 0.01 millimeter.
 5. The blank of claim 1 wherein the blankfurther comprises a fluoride releasing material.
 6. The blank of claim 1wherein the polymeric resin is made from a material comprising a freeradically curable monomer, oligomer or polymer.
 7. The blank of claim 1wherein the polymeric resin is made from a material comprising acationically curable monomer, oligomer or polymer.
 8. The blank of claim1 wherein the polymeric resin is made from a material comprising a freeradically curable monomer, oligomer or polymer and cationically curablemonomer, oligomer or polymer.
 9. The blank of claim 6 wherein thematerial is selected from the group consisting of2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]propane (bisGMA),triethyleneglycol dimethacrylate (TEGDMA),2,2-bis[4-(2-methacryloyloxyethoxy)-phenyl]propane (bisEMA), 2-hydroxyethyl methacrylate (HEMA), urethane dimethacrylate (UDMA) and anycombinations thereof.
 10. The blank of claim 7 wherein the material isselected from the group consisting of diglycidyl ether of bisphenol A,3,4-epoxycyclohexylmethyl-3-4-epoxy cyclohexene carboxylate, bisphenol Fepoxides, and polytetrahydrofuran.
 11. The blank of claim 1 wherein theresin is made from a material comprising a monomer, oligomer or polymercomprising both a free radically curable functionality and acationically curable functionality.
 12. The blank of claim 1 wherein thefiller is selected from the group consisting of barium glass, quartz andzirconia silica.
 13. The blank of claim 1 wherein the filler is derivedfrom a sol-gel process.
 14. The blank of claim 1 wherein the blank iscapable of being further hardened after or during milling by a curingprocess.
 15. A carvable mill blank for making a dental prostheticcomprising a) a resin component and b) a fluoride releasing component.16. A method of making the dental mill blank of claim 1 comprising thesteps of a) mixing a paste comprising a resin and a filler, b) shapingthe paste into a desired configuration, c) minimizing materialdiscontinuities from the paste d) curing the paste into a blank, and e)relieving internal stresses in the blank.
 17. The method in claim 16wherein shaping the paste is performed using a mold and furthercomprising the steps of f) trimming excess paste material from the mold,and g) removing the cured paste from the mold.
 18. The method in claim16 further comprising the step of f) mounting a handle to the curedpaste.
 19. The method in claim 16 wherein the curing system is selectedfrom the group consisting of heat, light, microwave, e-beam and chemicalcure.
 20. The method in claim 16 wherein the stress relieving stepcomprises slowly heating the cured paste in an oven temperature of at orabove Tg of the resin.
 21. A method of making the dental mill blank ofclaim 1 comprising the steps of a) mixing a paste comprising a resin anda filler, b) shaping the paste into a desired configuration, c)minimizing material discontinuities from the paste d) slow curing thepaste on a light box for a sufficient time to effectuate low stresscure, such that the cured paste passes a Thermal Shock Test.
 22. Amethod of making a dental prosthetic comprising the steps of a) mixing apaste comprising a resin and a filler, b) shaping the paste into adesired blank configuration, c) minimizing material discontinuities fromthe paste, d) curing the paste into a blank, e) carving the blank into adesired shape and morphology, wherein the blank is substantially free ofcracks and fabricated such that the blank passes a Thermal Shock Test.23. The method of claim 22 further comprising the step of: f) addingadditional material to the carved blank.
 24. The method of claim 22further comprising the step of: f) attaching the carved blank to toothor bone structure.
 25. The method of claim 22 further comprising thesteps of: f) manually changing the morphology of the carved blank and g)finishing the outer surface of the carved blank.
 26. The method of claim22 wherein an intermediate step between curing and carving the pastecomprises attaching a handle to the cured paste and wherein the carvingis performed by a milling machine.
 27. The method of claim 22 whereinthe carving step is performed by a hand-held instrument.
 28. The millblank in claim 1 wherein the wherein the mill blank has a BarcolHardness value greater than about 0% of the Barcol Hardness of aStandard Fumed Silica Mill Blank, and a Cuttability value greater thanabout 30% of the Cuttability value of a Standard Fumed Silica MillBlank.
 29. The mill blank in claim 1 wherein the mill blank has a BarcolHardness value greater than about 5% of the Barcol Hardness of aStandard Fumed Silica Mill Blank.
 30. The mill blank in claim 1 whereinthe mill blank a Barcol Hardness value greater than about 15% of theBarcol Hardness of a Standard Fumed Silica Mill Blank.
 31. The millblank in claim 1 wherein the mill blank has a Cuttability value greaterthan about 50% of the Cuttability of a Standard Fumed Silica Mill Blank.32. The mill blank in claim 1 wherein the mill blank has a Cuttabilityvalue greater than about 100% of the Cuttability of a Standard FumedSilica Mill Blank.
 33. The mill blank in claim 1 wherein the filler isat least about 50% by weight of the total weight of the mill blank. 34.The mill blank in claim 1 wherein the filler is at least about 65% byweight of the total weight of the mill blank.
 35. The mill blank inclaim 1 wherein the filler is at least about 80% by weight of the totalweight of the mill blank.